It is generally known that the effectiveness of oil-type drawing lubricants is, in large part, a function of their viscosity. It is also generally known that an extremely thin film (of the order of 50 microinch) of a relatively high viscosity lubricant is adequate to permit the fabrication of metal by severe stamping operations or the like if the lubricant possesses the other necessary physical properties. However, in order to coat surfaces with high viscosity lubricants, it has been necessary to apply them with the aid of steel rolls under high pressure. In spite of all efforts, it has seldom been possible for high viscosity lubricant films of less than 200 microinch in thickness to be applied in this manner. Most often, the resulting film thicknesses were around 1000 microinch or more in thickness. As a result, from around 4 to 20 or more times the necessary quantity of high viscosity lubricants has been applied in this manner at wastefully excessive material costs.
When flat, metal workpiece blanks are coated with viscous, oil-type lubricant films of 200 microinch or more in thickness, separation of the blanks for feeding them to the deforming operations is difficult. Therefore, since the application of thinner films has not been readily achieved or controllable heretofore, the use of high viscosity, oil-type lubricants on metals to be deformed has generally been restricted to applying the viscous lubricants to workpiece blanks immediately prior to placing the blanks into die cavities in which they are to be worked. This is undesirable because the excessive amounts of oil applied leave heavy, residual oil films on the formed metal parts. Such heavy residual oil films, if not removed, interfere with frequently required welding, adhesive bonding, and painting of the formed parts, and it is desirable, therefore, to produce the formed metal parts with as little residual oil as possible left on their surfaces. Moreover, the application of oil type lubricants immediately prior to their movement into deforming dies or the like creates serious housekeeping problems around the presses and requires much wasted time cleaning the die bed areas of oil before press adjustments and repairs can be made safely and efficiently.
Among the expedients heretofore employed to overcome the lubricant application problems discussed above is the dispersion of a viscous drawing lubricant in the form of an aqueous emulsion and applying the emulsion to the metal in the proper quantity to deposit the desired thin film of viscous lubricant on the metal surface. Good lubricant performance has been obtained in this way, but it is not practical to precoat the metal in this way at steel mills or in blanking operations at the place of use of the metal because the water of the emulsions causes troublesome corrosion of the metal prior to its use. Also, precoating the metal in this way at blanking lines causes dangerous load shifting of the stacked wet blanks and messy and dangerous housekeeping conditions around the blanking lines. The need for drying of the emulsions after their application precludes applying them to the blanks as the blanks are moved from stacks into the metalworking operations.
Another expedient heretofore employed to overcome the lubricant application problems discussed above is to apply the viscous lubricant as a solution of the lubricant in a volatile solvent which, upon subsequent evaporation, leaves the desired thin film of viscous lubricant on the metal surface. However, this creates a serious fire hazard and toxicity problems for personnel in the coating and drying areas, whether the operations are performed at the mill or at a later time. Furthermore, the space requirements for the coating and drying operations are generally not available either at the mill or at fabricating plants. Also, facilities normally not present at either type of plant are required and such facilities, therefore, are not easily made a part of existing operations of mills or fabricating plants in a cost effective manner. These latter problems are particularly acute at mills where metal sheet being produced is moving at high speed, sometimes in excess of 2,000 feet per minute, so that it is impractical to provide time for solvent evaporation after the metal has been coated and before it is coiled.
Still another expedient heretofore employed to overcome the lubricant application problems discussed above is to apply various specially selected viscous lubricants in solutions in a less viscous, nonvolatile, nonaqeous carrier liquid, such as a mineral oil of insufficient viscosity to serve, alone, as an adequate deep drawing lubricant. The lubrication obtainable from the carrier liquid alone can be significantly increased in this manner by dissolving any of a variety of selected, high viscosity, high pressure lubricants in the carrier liquid without increasing its viscosity to an unmanageable degree (i.e., to a degree that would tend to recreate the application problems sought to be overcome). However, so far as we are aware, such composition have not provided sufficiently low coefficients of friction in severe deep drawing operations commonly performed in the metalworking industry.
Yet another expedient heretofore employed to overcome the lubricant application problems discussed above is, first, to apply and bond a solid resin polymer coating to the metal to be worked, preferably at the steel mill, and subsequently to apply an overlay coating of a relatively low viscosity oil or the like immediately prior to fabrication of the metal, as taught in U.S. Pat. No. 3,568,486. In accordance with that patent, the resin polymer coating and overlay coating are selected so that the latter coating softens the former coating through part of its thickness without impairing its bond to the metal, and the softened portion of the coating serves as a high pressure lubricant while the unsoftened portion, still bonded to the metal, protects the surface of the metal from scuffing. Although that process has been widely used with outstanding success in protecting the metal from corrosion and surface marring prior to fabrication and in providing lubrication during severe deep drawing operations, it is a relatively costly process because the resin polymer is expensive and two separate coating steps are required. Moreover, the process of that patent is also not practical for use in mills in which metal being rolled is moving at high speed.